Individual laser emitters are typically limited in output power due to thermal effects which degrade the laser efficiency, distort the wavefront, and limit the beam quality. Limits on individual emitter performance can be mitigated at least in part by coherently combining several of the outputs from many emitters to form a single high-power beam. This conventional technique, which is known as coherent beam combining (CBC), is an attractive way to achieve high output power in a single diffraction-limited beam.
There are two types of conventional CBC: (1) tiled aperture CBC and (2) filled aperture CBC. In a tiled aperture configuration, also known as a side-by-side configuration, mutually coherent beams from multiple emitters propagate parallel to each other to form a single beam with a small divergence angle and a large area. In practice, however, there are often gaps between adjacent beams-in the other words, this configuration suffers from a low fill factor. Unfortunately, these gaps yield undesired sidelobes in the far field.
In a filled aperture configuration, a grating or coupler combines beams from multiple emitters to form a single output beam. When combining N beams, the coupler acts as an N-port beamsplitter: if the input beams are mutually coherent, then constructive interference yields the coherent output beam at one port, and destructive interference yields little to no output power at the other ports. The degree of mutual coherence limits the amount of power that can be coupled into the coherent output beam.
The emitters in a filled aperture configuration may be mutually cohered using passive feedback from a resonant cavity. Each element emits light at a finite number of discrete longitudinal modes, and only some of these lines resonate within the cavity, which also supports a finite number of discrete modes. Ideally, the cavity and the emitters support only one common mode; the other emitter modes do not resonate within the cavity. As a result, the output power of the emitters is coherently coupled into the common mode to produce a high-power output beam. Because each of these transverse modes has a finite bandwidth and associated optical path length, however, the likelihood of supporting only one common mode goes down as the number of emitters goes up. This imposes a practical limit on the number of output beams that can be coherently combined using passive feedback in a resonant cavity. In some examples, the limit is only seven or eight output beams, which curtails the maximum possible power of the coherently combined beams. For more information on this limit, see Dmitrii Kouznetsov et al., “Limits of Coherent Addition of Lasers: Simple Estimate,” Optical Review, Vol. 12, No. 6 (2005) 445-447, which is hereby incorporated herein by reference in its entirety.